Abstract
High speed cutting of die steels (A2, HRC58, 15 000 rev min-1, 385 m min-1) demands high wear and oxidation resistant and extremely well adherent coatings. PVD is the preferred deposition technique to tailor a coating for this application. With the development of TiAlN based hard coatings, an important step in the direction of improved cutting performance has been achieved. TiAlCrYN is a state of the art coating, which has proven to be very successful in demanding dry high speed cutting operations. A special feature of this coating is a double layered overcoat consisting of a lower fine multilayered TiAlYN/CrN with a periodicity of 1·6 nm graded into an upper glassy amorphous TiAlY oxynitride. The incorporation of Y leads to an enhanced oxidation resistance, whereas the most important effect of the double layered overcoat is the reduction of the friction coefficient. To confirm the beneficial effects of Y and Cr, sophisticated TEM has been employed with an X-ray resolution of approxi mately 3 nm. Segregation of Y and other elements in this system (tool/coating) were detected along the column boundaries in the TiAlCrYN coating at 900°C. Y is acting like a ‘plug’ against both inward and outward diffusion of foreign elements. The oxidation behaviour has been further quantified using highly sensitive thermogravimetric analysis and glancing angle X-ray diffraction. The successful impact of the incorporation of Y into TiAlCrN and the use of a Cr containing nanostructured oxynitride sputter deposited overcoat is shown by coated two flute ball nosed solid carbide cutters at a cutting speed of 385 m min-1.
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